In the production of semiconductor chips, as packing density continually increases the widths of the individual features become ever smaller. Corresponding to these smaller and smaller structures, requirements are becoming more stringent in terms of the specifications of coordinate measuring instruments that are used as measurement and inspection systems for measuring feature edges and feature positions, and for measuring feature widths. Optical sensing methods are still favored for these coordinate measuring instruments, even though feature widths are already smaller than the light wavelength used for measurement and inspection. The reason is that measurement systems with optical sensing methods are substantially easier to use than systems with other sensing methods, for example using electron beams.
Because the features being measured are becoming smaller and smaller, however, the requirements in terms of optical system performance, especially resolving power, become more stringent. For example, to allow optically reproducible measurement of feature widths, edge profiles, or the like, the measurement fields must be illuminated as homogeneously as possible.
Illumination devices that operate with an optical waveguide to generate a homogeneous illumination field are known. One such illumination device is used, for example, in the Leica® LMS IPRO coordinate measurement instrument of Leica Microsystems AG.
In this illumination device, the light of a light source is picked off via a coupling-in optical system with a small numerical aperture (e.g. NA=0.18) and coupled into a multimode optical waveguide. The light source used is a 100 W Hg—Xe discharge lamp. The multimode optical waveguide possesses a small core diameter of 0.4 mm and a nominal numerical aperture NA=0.21. The specimen is then Köhler-illuminated in known fashion via a coupling-out optical system, using a tube lens and a PLAN-APO 50× objective. An image field with a diameter of 0.056 mm is illuminated with this illumination device. To achieve uniform illumination with inhomogeneities of approximately a few percent in this context, only a small numerical aperture NA=0.12 is picked off by the multimode fiber. This corresponds to a semiangle of approx. 7°. As a result, only that region of the fiber's emission characteristic curve in which the intensity of the light varies by only a few percent as a function of emission angle is used.
The known illumination device has the advantage that the light source, which at the same time is a strong heat source, can be arranged well outside the actual measurement location and at a great distance from the specimen being measured. Temperature influences on the measurement accuracy of the coordinate measuring instrument are thereby minimized. At the same time, the illuminating light is conveyed via the flexible optical waveguide to any desired measurement location.
The disadvantage of the known illuminating device is that because of the emission characteristic curve of multimode fibers, the angular region in which the intensity of the emitted light varies only on the order of a few percent is very narrow. If larger fields nevertheless need to be homogeneously illuminated, the fiber diameter must be correspondingly large. The rigidity of optical fibers rises sharply with increasing diameter. For future coordinate measuring instruments, for example, a homogeneously illuminated image field of 0.35 mm is necessary. This would require the use of an optical fiber having a core diameter greater than 1 mm in order to achieve sufficient homogeneity in the image field. Fibers with a core diameter greater than 1 mm are, however, no longer practically usable because of their rigidity.